Magnetic recording has an increasing range of application for audio and video recording as well as various information processing purposes. The important factors imposed on the magnetic recording media for such magnetic recording are that original signals can be recorded and reproduced as faithfully as possible and that they can store in a limited area as much information as possible. Efforts have been made for the development of improved magnetic recording medium and magnetic material in order to achieve the purposes of high fidelity, high reliability and high recording density. A similar situation is found with respect to the magnetic material intended for use in a recording layer of thermomagnetic recording media capable of high density recording such as magnetooptical recording media.
The recording wavelength should be reduced in order to increase the recording density of a magnetic recording medium. As the recording wavelength is reduced, the size of a pit being recorded becomes substantially smaller than the thickness of the medium, approaching to the size of magnetic particles. Then isotropic or perpendicular magnetizable components provide a greater contribution than longitudinal magnetizable components of the medium. For this reason, particulate or spherical particles tending to orient in an isotropic or perpendicular direction are more advantageous for high density recording than acicular particles tending to orient in a longitudinal direction of the medium.
In the manufacture of magnetic disks of the flexible type, magnetic particles are applied to a length of film stock in a longitudinal direction. The coated film is finally punched into disks which are required to have circumferentially uniform properties. For such uniformity of properties, particulate or spherical particles tending to remain randomly oriented independent of a direction of application are more advantageous than acicular particles tending to orient in a direction of application.
For magnetooptical recording medium wherein recording is generally effected in a perpendicular direction to the medium surface, particulate or spherical particles are preferred for the same reason as above.
It is thus recognized from these points of view that flakes of Ba ferrite base material are superior recording material for high density recording. Flakes of this material, however, suffer from many drawbacks that they tend to be electrically charged because they are dielectric oxide particles, that they are formed into a coating having an increased electrical resistance as the magnetic layer, and that they tend to agglomerate without dispersing because they are synthesized through heat treatment at a relatively high temperature. In addition, the flake Ba ferrite material cannot be used for optical and thermomagnetic recording because its Curie temperature is 400.degree. C. or higher.
Nevertheless, most magnetic particles currently used for magnetic recording are of acicular or needle shape. This is partly because they have an increased coercive force. Particulate or spherical form of metallic iron (.alpha.-Fe), for example, is not expected to have a high coercive force because its magnetocrystalline anisotropy is low. Thus acicular .alpha.-Fe magnetic powder whose coercive force has been increased by imparting magnetic shape anisotropy is commonly used.
Metallic cobalt has a stable hexagonal structure at room temperature and relatively high magnetocrystalline anisotropy. Metallic cobalt is thus expected to produce a high coercive force even in particulate or spherical particle form if it is possible to take advantage of the high magnetocrystalline anisotropy. However, it is known from B. Szpunar, J. Magnetism of Magnetic Materials, Vol. 49 (1985), page 93 that if metallic cobalt is finely divided to a sufficient size as recording material, a face-centered cubic structure becomes stable from an energy aspect rather than a hexagonal structure affording high magnetic anisotropy. For this reason, none of currently available cobalt particles have a high coercive force as expected.
For these reasons, metallic magnetic materials based on iron or cobalt are finely divided into an acicular shape or chain structure so that an increased coercive force is available due to a shape effect.
As opposed to the metallic magnetic powders, hexagonal intermetallic compounds of Fe-Co-P series are expected to develop a high coercive force in particulate or spherical particle form because of high magnetocrystalline anisotropy.
Japanese Patent Publication No. 4755/1963, for example, discloses a permanent magnet comprising a three-component intermetallic compound (Fe,Co).sub.2 P having a hexagonal crystalline structure wherein the Fe:Co ratio is up to 60:40 and the phosphorus content is from 20 to 22%.
According to this publication, the intermetallic compound is manufactured by first preparing a molten bath of phosphorus-copper (Cu 90%, P 10%). Predetermined weight proportions of electrolytic iron powder and cobalt pieces are added to the molten bath to form a bath consisting of 95% of phosphorus-copper and 5% of iron-cobalt. The molten metal is then poured into a steel mold to form a casting. The casting is leached with a weak or strong acid, obtaining (Fe,Co).sub.2 P. The intermetallic compound (Fe,Co).sub.2 P is available in fine particulate form according to the description of said publication. However, the compound cannot be obtained in a ultrafine particle size as defined in the present invention without an additional comminuting step. In addition, mechanical comminution generally gives particles having a poor particle size distribution and a low degree of sphericity and is difficult to produce ultrafine particles of 0.1 .mu.m or smaller in size.
Japanese Patent Publication No. 5757/1964 discloses an improvement in the manufacture of the intermetallic compound (Fe,Co).sub.2 P described in the above-cited Japanese Patent Publication No. 4755/1963 wherein the cooling rate at which the molten metal is cooled in the mold is controlled so as to increase the coercive force. The thus produced intermetallic compound particles are reported to exhibit a coercive force of 735 Oe at the maximum, much lower than a coercive force in excess of 2,000 Oe as achieved by the present invention with the equivalent composition, that is, stoichiometric composition.
In order that the intermetallic compound (Fe,Co).sub.2 P produced by the method of Japanese Patent Publication No. 5757/1964 exhibit a coercive force comparable to that of the ultrafine particles of the present invention, the method should involve an additional heat treatment as taught in said publication, resulting in an overall complicated process.
Besides, the intermetallic compound (Fe,Co).sub.2 P described in the above-cited Japanese Patent Publication Nos. 5757/1964 and 4755/1963 is less resistant to corrosion and prone to a loss of magnetic properties. In this regard, Japanese Patent Publication No. 11085/1983 proposes to improve the corrosion resistance of the intermetallic compound (Fe,Co).sub.2 P by adding Cr thereto. It discloses ferromagnetic particles comprising hexagonal metal phosphide of the formula: EQU (Fe.sub.(1-x-y)Co.sub.x Cr.sub.y).sub.2 P
wherein 0.045&lt;x+y&lt;0.40 and 0.005&lt;y&lt;0.10, the particles having a mean particle size of 0.02 to 1.0 .mu.m.
These ferromagnetic particles are produced by substantially the same method as the intermetallic compound (Fe,Co).sub.2 P particles described in the above-cited Japanese Patent Publication Nos. 5757/1964 and 4755/1963. An extra comminuting step must be added before ferromagnetic particles having a mean particle size of 0.02 to 1.0 .mu.m can be available.
As described in Japanese Patent Publication No. 11085/1983, the ferromagnetic particles exhibit a coercive force of 200 to 500 Oe despite the inclusion of P in the stoichiometric amount. An additional heat treatment is necessary in order to increase the coercive force to 2,000 Oe.
Some known methods for the preparation of ultrafine particles of metal and alloy materials utilize a gas phase process. Known gas phase processes are vacuum evaporation processes as disclosed in Japanese Patent Publication Nos. 5149/1975, 5665/1975, 5666/1975, 21719/1977, and 44123/1980 and Japanese Patent Application Kokai Nos. 55400/1973, and active plasma arc evaporation processes as disclosed in Japanese Patent Publication No. 44725/1982 and Japanese Patent Application Kokai Nos. 104103/1983 and 162705/1985.
Basically, these processes are devised to heat a source ingot metal to a high temperature with an electron beam, arc plasma or the like to evaporate atoms from the metal surface, followed by condensation of the vapor for collection.
When these gas phase processes are applied to the manufacture of fine particles of hexagonal intermetallic compound of Fe-Co-P series as previously mentioned, several problems arises. First, it is difficult to continuously form a compound or composite material from such elements having greatly different melting and boiling points as Fe, Co, and P since a source ingot is melted and evaporated. Secondly, an alloy ingot used as the evaporation source is expensive.
One solution to the first problem is Japanese Patent Application Kokai No. 149705/1985, which is successful in forming a composite material from elements having different boiling points. However, this process is unsuitable for mass production because an apparatus of complex structure is necessary. In addition, this process is presumed difficult to produce hexagonal Fe-Co-P intermetallic compounds, especially ultrafine particles of hexagonal Fe-Co-P intermetallic compounds within the composition range defined in the present invention.